Known as a conventional laser processing method is one irradiating a planar object to be processed with processing laser light while locating a converging point within the object, so as to form a modified region to become a cutting start point within the object along a line to cut the object (see Patent Literature 1, for example).
For accurately forming a modified region at a desirable position with respect to a laser light irradiation surface of the object, there is a case where such a laser processing method irradiates the laser light irradiation surface along the line to cut with measuring laser light for measuring the displacement of the laser light irradiation surface and detects the light reflected by the laser light irradiation surface in response to this irradiation.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-343008